1. Field of the Invention
The present invention relates to rubber compositions. More particularly, the present invention relates to improvements in the rate of cure and the properties of vulcanized products such as tensile strength when sulfur vulcanization of known elastomers containing unsaturated double bonds is carried out, and to rubber compositions having excellent co-vulcanization ability when blended with other rubbers.
2. Description of the Prior Art
In general, sulfur vulcanization has been widely practised in the rubber industry. Commonly, vulcanization has been carried out by mixing an unsaturated elastomer together with sulfur, a metal oxide such as ZnO, CdO, MgO, PbO, CaO or NiO, etc., fatty acids, vulcanization accelerators and another additives for rubber compositions, and pressing with heating. Vulcanization of this type is believed to proceed as follows. ##STR1##
The above described reactions are one model of vulcanization. Practically, these reactions are accompanied by other very complicated reactions. Thus, it has been believed that such reactions form heterogeneous cross-linking points. Accordingly, the properties of the rubber can be further improved in a vulcanization of such a type, if a more homogeneous vulcanization is carried out. It is also known that low unsaturated rubbers such as EPDM and IIR have a good weather-proofing property and particularly good ozone cracking resistance, because such rubbers have a small number of double bonds in the structure. However, these rubbers have essential defects in that they have poor tackiness and thus the molding or processing involved in producing products which require complicated workings such as tires and belts, etc., are difficult to carry out and also sulfur vulcanization which has a rapid rate of cure is difficult to carry out because of the presence of a small amount of unsaturated bonds. At the present time, even though many studies have been made to improve these defects, satisfactory results have not been obtained.
On the other hand, high unsaturated rubbers such as NR and SBR have essential defects in that they have a poor heat resistance and a poor weather-proofing property because of a large number of double bonds (unsaturated groups) in the structure.
For the purpose of improving the weather-proofing property of these highly unsaturated rubbers, production of blends of these rubbers and the above described low unsaturated rubbers has been attempted. However, although blends having a certain mixing ratio wherein one of the two rubbers is present in a small amount can be practically used, a practical blend of both rubbers at a suitable ratio has not been achieved, because they form a heterogeneous phase resulting in a deterioration of properties, since both rubbers have essentially different properties. The reasons are that there is no interaction between the two rubbers because they are incompatible with each other and that it is difficult to carry out homogeneous vulcanization of the blend because each rubber has a different rate of cure on sulfur vulcanization. Accordingly, many studies have been made in order to obtain good co-vulcanized products by blending these rubbers.
Namely, the following approaches have been suggested:
(1) to increase the amount of sulfur added,
(2) to vulcanize using peroxides,
(3) to increase the amount of the third component (ethylidenenorbornene) so as to increase the rate of cure of EPDM itself,
(4) to modify EPDM with halogens, and
(5) to blend a slightly pre-vulcanized EPDM so that the cure rates of the rubbers more closely approach each other.
However, these methods are not practical. A good result can be obtained only where EPDM is modified by bromine. However, this case is suitable only for a special use, because the brominating agent as the modifier is expensive, the bromine is dangerous to humans, and storage ability is inferior because the vulcanization of the brominated EPDM itself proceeds at room temperature by metal oxides or amines after mixing. Consequently, it is not actually used as an available rubber.